Amperage control for protection of battery over current in power tools

ABSTRACT

Amperage control of a power tool motor is provided by pulse width modulation of current from a power supply. The pulse width modulation may be varied according to the determined motor current and measured power supply voltage. The power supply preferably includes a battery, such a lithium ion or nickel cadmium.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119 of U.S.Provisional Application No. 60/448,267 filed Feb. 18, 2003. Said U.S.Provisional Application No. 60/448,267 is incorporated by reference.

FIELD OF THE INVENTION

The present invention generally relates to the field of power supplymanagement for power tools, and particularly to a method and apparatusfor extending battery life in a power tool.

BACKGROUND OF THE INVENTION

It is a concern in the power tool industry to maximize battery life andto prevent battery damage. Further, it is desirable to maximize motorlife and prevent motor damage. Typical cordless power tools use nickelcadmium (NiCd), nickel metal hydride (NiMH) or lithium ion batteries anddraw 30 to 35 or more amps while under heavy load. Serious degradationof NiCd battery life occurs at or above 50 amps of current. While underheavy loads, near stall or when a battery is nearly depleted, the powertool may draw in excess of 50 amps. Other battery types have differenttolerances for high current. For example, above 15 amps, damageassociated with temperature may occur to state of the art lithium ionbatteries.

Thus, it would be desirable to provide a circuit to limit current from abattery to a motor in a power tool.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a method and circuitfor limiting current to a motor in a power tool.

In a first aspect of the present invention, a method for extending thelife of a battery is presented that supplies power to a motor, such as apermanent magnet direct current motor in a power tool. The method forextending battery life may utilize a pulse width modulation controllerto supply voltage pulses of an initial width to the permanent magnetdirect current motor. The width of the pulse may be altered from aninitial width to an alternative width if the current changes beyond athreshold.

In a second aspect of the present invention, the power tool of thepresent invention may include a power supply, a motor for providingtorque to a power tool, and a pulse width modulation controller fordetermining an amount of voltage supplied from the power supply to themotor. Power tools may include drills, saws, sanders, grinders, routers,and the like that employ electrical power to an electric motor.

This invention not only protects batteries, it also enables power toolsto use batteries where the potential current exceeds the maximumallowable current of the batteries.

It is to be understood that both the forgoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention as claimed. The accompanyingdrawings, which are incorporated in and constitute a part of thespecification, illustrate an embodiment of the invention. Together withthe general description, these drawings serve to explain the principlesof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The numerous advantages of the present invention may be betterunderstood by those skilled in the art by reference to the accompanyingfigures in which:

FIG. 1 illustrates an embodiment of a method of the present invention;

FIG. 2 illustrates a basic block diagram an embodiment of the method ofthe present invention;

FIG. 3 illustrates a functional block diagram of an embodiment of thepresent invention;

FIG. 4 illustrates an embodiment of an exemplary circuit of the presentinvention;

FIG. 5 illustrates an embodiment of an analog-to-digital conversionblock of FIG. 4;

FIG. 6 illustrates a functional block diagram of another embodiment ofthe present invention;

FIG. 7 illustrates a functional block diagram of yet another embodimentof the present invention;

FIG. 8 illustrates illustrative pulse width modulation waveforms of thepresent invention;

FIG. 9 illustrates an exemplary embodiment of pulse width modulationgeneration circuitry;

FIG. 10 illustrates a graph of voltage versus current for a lithium ionbattery power source of the present invention; and

FIG. 11 illustrates a graph of power, torque, efficiency, and rpms forthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the presently preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings.

The present invention relates to a method and apparatus for controllingcurrent supplied to a motor of a power tool, especially, a hand heldpower tool such as a drill. The present method may rely on pulse widthmodulation to control a voltage supplied to the motor, whichproportionately affects the current supplied to a motor; especially, toa permanent magnet direct current motor. In one embodiment, circuitry ofthe present invention may determine motor current and battery voltage ata determined sampling rate. The measured values may be supplied to amicroprocessor or other circuitry that pulse width modulates a voltagesupplied to the power tool driving motor. In another embodiment, analogcircuitry switches power to the motor according to the amount of currentbeing provided to the motor.

FIG. 1 illustrates an embodiment of a method 5 of the present invention.Method may start 10 by measuring current 15. If the current is below athreshold level A in step 20, the duty cycle may be checked to determineif it is at a 100% level (step 22). If the duty cycle is at a 100%level, the method may repeat. If the duty cycle is less than 100%,voltage may be raised (step 25). To prevent damaging a battery of apower tool, whenever the set amperage is exceeded (step 35, current isabove threshold level B), the voltage is reduced (step 40). The voltageis reduced to the limit set by the controller. A pulse width modulation(PWM) controller reduces the average voltage so that the stall currentof the motor will not exceed the limit of the batteries. Then, as theload is removed from the motor, the PWM controller raises the voltageuntil it reaches nominal peak voltage as long as the current stays belowthe limit. In an embodiment of the invention, the raising and reducingof the voltage refers to the voltage level of the overall averagedvoltage.

A basic functional block diagram of an embodiment of an apparatus 200 ofthe present invention is shown in FIG. 2. This invention preferably usesa pulse width modulation controller 220 with a closed amperage (orcurrent) feedback loop, a permanent magnet direct current (PMDC) motor230, and a power supply 210. The power supply 210 of the presentinvention is preferably a battery or a series of batteries. Lithium ionmay be a preferred type of battery. For lithium ion batteries, a maximumsafe current may be approximately 15 amps, however, current limits maybe affected by the battery temperature when driven by the amp load.Other batteries may be used, such as hydrogen ion batteries, nickelcadmium batteries, and the like. The batteries are preferablyrechargeable.

Permanent Magnet Direct Current (PMDC) motors are durable motors whosepermanent magnet field is able to operate for thousands of hours andfunction in long duty cycle applications. These motors also have a longshelf life and high-stall torque. PMDC motors produce maximum torque atzero rotations per minutes (RPMs), produce zero torque at the maximumRPMs, and produce 50% of maximum torque at 50% of the maximum RPM value.At 50% of the maximum RPM value, the PMDC motors produce maximumhorsepower. PMDC motors, at any given voltage, consume amps proportionalto the torque required. Actual stall currents of PMDC motors in powertool applications are limited by impedance of the battery pack toapproximately 85 amps for NiCd, regardless of the designed stall currentof the motor. Other combinations of motors, batteries, and controllersmay be used in the present invention.

An exemplary embodiment of a current feedback loop 300 is shown in FIG.3. A battery voltage 310 is supplied by the battery. The currentsupplied by the power source is increased or decreased through a voltageadjustment 320. The measured current 330 is provided to themicroprocessor 325 which determines if the current is to be increased ordecreased, as controlled by the voltage adjustment 320. Current, withina desired range, may be supplied to the motor 350, such as a PMDC motor.

FIG. 4 illustrates a circuit 400 of an exemplary embodiment of thepresent invention. The power supply 410 may use lithium ion batteriesfor low current applications, hydrogen ion batteries for even lowercurrent applications, NiH6 batteries when more stable current operationsare needed, or batteries of another type. The power supply 410 ispreferably two or more batteries, always in series, such that eachbattery serves as one component of a multi-component power supply. Thebatteries are arranged such that the positive terminal that is uncoupledto any other battery serves as the positive terminal of the power supplyand the negative terminal that is uncoupled to any other battery servesas the negative terminal of the power supply. Preferably, the batteriesare rechargeable. Other power supply arrangements are contemplated bythe present invention.

A manually operated switch SW1 is preferably used to electricallyconnect the power supply 410 with the circuitry. Power supply voltage iscontinually monitored once the circuitry is activated by closing switchSW1. A voltage divider in parallel with the power supply 410 permitsmeasurement of voltage over time for monitoring power levels of thepower supply 410. The voltage measured by the analog to digitalconversion block 420 is determined by a voltage divider formed ofresistors R1 and R2. For a battery voltage of approximately 40 volts, R1may be set to a value seven times greater than the value of R2 toprovide an approximate maximum 5-volt value to the A/D conversion block420. The resistors may be selected to minimize the current drawn throughthe voltage divider. For instance, R1 may be 100 kΩ and R2 may be 14.3kΩ. Preferably, the resistors used in the voltage dividers aretemperature insensitive and precision resistors.

The pulse width modulation controller determines the current supplied tothe motor 470. The amount of current supplied may be determined by aswitch such as a power transistor 430. The power transistor base (for abipolar junction transistor) or gate (for a metal oxide semiconductorfield effect transistor) is supplied a signal by the microprocessor 440.This signal may be set at a high voltage level for turning on the powertransistor 430 and may be set at a low voltage level for turning off thepower transistor 430; in other words, in a binary manner. Alternatively,the converse arrangement of a low voltage level turning on the powertransistor may be employed. To ensure adequate voltage is delivered tothe power transistor 430, a booster circuit 490 may raise the voltage orprovide adequate driving capability for switching the power transistor.The booster circuit may be an operational amplifier, a transistor, orthe like. The microprocessor determines the signal to be supplied bymonitoring battery voltage and motor current. The power transistor mayinclude two or more power transistors in parallel and may also include adiode to enhance reliability. An advantage to using a power transistoris that the power transistor dissipates minimal power. The pulse widthmodulation power transistor 430 may turn on, for a percentage of timeregulating average voltage to the motor and thereby limiting the averagevoltage to the motor when peak currents are exceeded. The duty cycle ofthe pulse width modulation is varied according to an algorithm providedin the executable code of the microprocessor. When the power transistoris on for longer periods, the voltage provided to the motor increases,thereby increasing motor speed. The rate of pulse generation may bevaried so as to ensure accurate and adequate current flow to the motor.The power transistor, in an alternative embodiment, may be placedbetween the motor and ground.

As illustrated in FIG. 4, a filter may be provided at the output of thepower transistor 430. Here, an inductor L1 dampens current spikes and acapacitor C1 smoothes out the voltage to the motor. Many differentfilters may be used to smooth out the pulsed voltage waveform. Thetransistor 430 is an option as the power transistor may be pulsed oftenenough that the inductance of the motor compensates for any abruptvoltage changes across the motor.

A current measurer 455 may be used to determine current through themotor 470. Flyback diode 460 and/or capacitors may be disposed acrossthe terminals of the motor to smooth out current changes, to suppressvoltage spikes, and to provide a current path when power is cut off. Thecapacitors may be of a suitable type and value, such as 0.1 μFd ceramiccapacitors. Diode 460 may be a single diode or multiple diodes.

Motor speed control may be manually set by a rotary knob externallyaccessible to an operator on the outside of the power tool housing. Forexample, the rotary knob may be coupled to a potentiometer R6 thatcontrols a value of a resistor in a voltage divider with anotherresistor R5. The voltage across the potentiometer resistor R6 isdigitized by analog-to-digital (A/D) conversion block 480. The digitizedvoltage value is then read by the microprocessor 440 to determine themotor speed setting.

The microprocessor 440 provides analysis and control in the pulse widthmodulation speed controller. The microprocessor may have an associatedread only memory (ROM) and an associated random access memory (RAM). Inthe embodiment of FIG. 4, a microprocessor 440 controls the pulse widthmodulation of the motor 470. The microprocessor 440 is loaded withexecutable code from read only memory. The executable code provides formeasuring the battery voltage, the motor voltage, and a speed controlpotentiometer voltage. The executable code may be written so as topermit different kinds of batteries in power supply 410. A switch on thebody of the power tool may be implemented so as to allow a user tochoose a particular type of battery. The microprocessor would read theswitch value to determine the portion of the executable code to run fora particular battery type. The power supply housing may also containactuators that are able to detect the battery type placed within. Anexample of the processing performed by the microprocessor 440 isprovided by the following pseudo code:

-   Measure battery voltage;    -   Address battery voltage storage;    -   Read battery voltage value;    -   Store battery voltage;-   End battery voltage;-   Measure motor current;    -   Address motor voltage storage;    -   Read motor voltage;    -   Calculate motor current;    -   Store motor current;-   End motor current;-   Measure speed control potentiometer voltage;    -   Address speed control potentiometer voltage storage;    -   Read speed control potentiometer voltage;    -   Store speed control potentiometer voltage;-   End speed control potentiometer voltage;-   Adjust pulse width;    -   Case (motor current>maximum tolerable current) then pulse        width=0 and shut off power to the motor;    -   Case (motor current>upper current level 1 and motor        current≦maximum tolerable current) then pulse width=a;    -   Case (motor current>upper current level 2 and motor        current≦upper current level 1) then pulse width=b;

And so forth;

-   -   Case (motor current>upper current level n and motor        current≦upper current level n−1) then pulse width=n;

-   End Adjust Pulse Width;

-   Main

-   Begin_loop;    -   Measure battery voltage;    -   Measure motor current;    -   Measure speed control potentiometer voltage;    -   Scale factor=speed control potentiometer voltage/C;    -   Case (motor current<threshold a) then adjust pulse width;    -   Case (motor current>threshold b) then adjust pulse width;    -   Case (battery voltage<threshold c/Scale factor) then issue alert        and shut off power to the motor;    -   Case (battery voltage>threshold d/Scale factor) then issue alert        and shut off power to the motor;

-   End_loop;

-   End Main;

It is contemplated that stages produced for varying increasing motorcurrent values may also be applied to decreasing current values.

The power supply for the microprocessor, A/D converters, and boostercircuit may be provided by the power supply 410 or another power source,such as a separate battery. A monitoring circuit may independentlymeasure the strength of power supply 410 or may receive a signal fromthe microprocessor 440 to determine this value. If the monitoringcircuit determines that the power supply 410 has been excessivelydischarged, the microprocessor and related circuitry may be switched toa back up battery to continue operations. Also, an indicator light, suchas a light emitting diode, may be lit on the power tool housing.Alternatively, a display on the housing may provide accurate readings ofthe power supply conditions of the power tool circuitry.

FIG. 5 illustrates an exemplary embodiment of an A/D conversion block500. A/D conversion block may be representative of A/D converter block420, 450, 480 of FIG. 4. The A/D converter 510 receives the analogvoltage input directly from a voltage measuring point, such as from avoltage divider, through an operational amplifier, or the like. The A/Dconverter 510 provides a digitized value to a tristatable latch 520. Foreach sample clock pulse, a digitized value from the A/D converter 510 isstored in the latch 520. The sample clock may be provided by themicroprocessor, from an oscillator circuit, or by other circuitry. Whenthe microprocessor 440 executes the code that requires the currentdigitized value, the microprocessor provides the address for the latchstoring this value. In other embodiments, the number of address bits mayvary, such as having three address bits to address from five to eightaddressable latches. In the present embodiment, a two-bit address buswould be sufficient to uniquely address the three A/D conversion blocks.The AID conversion block's addressing logic enables the tristatablelatch 520. A read enable signal may also be provided to read data fromthe tristatable latch 520. The read enable signal is optional. In oneembodiment, the address logic serves to disable the sample clock and theread enable in conjunction with the address logic serves to permit thelatched digitized value to be read from the bus.

FIG. 6 illustrates a functional block diagram of another embodiment of acircuit 600 of the present invention. An adjustable resistance 630 isplaced in series with the power supply 640 to better control the currentand voltage from the power supply. The voltage of the adjustableresistance 630 and the power supply 640 may be measured by a voltagemeasurement circuit 650. A current measurement circuit 620 may alsoprovide a current measurement to a microprocessor 610. Themicroprocessor 610 may signal the pulse width modulation controller 670to pulse on or pulse off. The pulse width modulation controller, as inthe other embodiments, may control the current and voltage supplied tothe motor 680. The adjustable resistance 630 may be used to offset achange in the internal resistance of the power supply 640.

FIG. 7 illustrates a functional block diagram for another embodiment ofthe circuit 700 of the present invention. This embodiment is especiallyuseful for an analog implementation of the present invention. A rampgenerator 720 is used to supply a ramp voltage, such as a saw tooth,triangular, staircase, or similar waveform. A voltage corresponding to acurrent across the motor 760 is compared with the ramp voltage by acomparator 730. A variety of processing algorithms may be employedthrough circuitry to supply the motor voltage value to the comparator.For instance, for a lithium ion battery power supply that operates inaccordance with FIG. 10, the switch may be initially opened withoutinterruption and, only when a certain motor current value correspondingto the measured motor voltage is reached, the switch may pulse the powerfrom the power supply to the motor. The pulse width during pulsing mayfurther be determined by the amount the determined motor current valueis above a certain current level. For example, the power supplied to themotor may be continuous up through a determined motor current of 14.2amps, then it may be supplied to the motor through pulses withadjustable duty cycle, and, if a certain motor current level is exceeded(e.g., 15 amps), the power supply may be shut off. A booster circuit 740may be used to provide ample current and voltage to actuate the switch750.

FIG. 8 illustrates exemplary pulse width modulation waveforms. Aninitial duty cycle may be set, such as a 50% duty cycle when implementedin software. A hardware implementation will quickly establish thenecessary duty cycle. When more voltage is to be supplied to the motor,the duty cycle may be adjusted to provide voltage for a largerpercentage of the cycle. Conversely, when less voltage is to be suppliedto the motor, the duty cycle may be adjusted to apply voltage for alesser percentage of the cycle.

FIG. 9 illustrates an exemplary embodiment of pulse width modulationcircuitry. A ramp signal is applied to the inverting terminal of anoperational amplifier. A voltage signal is applied to the non-invertingterminal of the operational amplifier. The voltage signal may beprovided directly from a voltage divider, may be further amplified orscaled, or may be otherwise provided by circuitry. The ramp signal maybe a saw tooth or triangular waveform. An integrator may also be used togenerate the ramp signal.

FIG. 10 illustrates the relationship of input 1010 to output voltage ofa controller for a lithium battery power supply of the presentinvention. As shown in the graph, the input power supply voltage 1010correlates in a one to one relationship with the controller outputvoltage 1020 until about 14 amps in which both input and outputcontroller voltages decline in a gradual linear manner. At slightlyabove 14 amps and above, the output controller voltage decreasesrapidly.

FIG. 11 illustrates the relationship of motor efficiency 1140, torque1130, power consumed 1110, and rotations per minute of the motor 1120.

While the present application has disclosed a method and system forreducing power, it is contemplated that other types of mechanisms may beemployed to reduce voltage and/or current to reduce power which wouldnot depart from the scope and intent of the present invention. Theembodiments disclosed herein for reducing power being only exemplary ofthe present invention.

Other variations are contemplated by the present invention. Opticalencoders, clamped on ammeters, tachometers, protection circuits,emergency power cut off, current sensing transducers, voltage sensingtransducers, current shunts, error amplifiers, clipping circuits, andthe like may be used. The pulse width modulation switch is not limitedto being a transistor, but may be another component, such as amechanical relay switch.

It is believed that the present invention and many of its attendantadvantages will be understood by the forgoing description. It is alsobelieved that it will be apparent that various changes may be made inthe form, construction and arrangement of the components thereof withoutdeparting from the scope and spirit of the invention or withoutsacrificing all of its material advantages, the form hereinbeforedescribed being merely an explanatory embodiment thereof. It is theintention of the following claims to encompass and include such changes.

1. A method for extending the life of a power supply that supplies powerto a motor in a power tool, comprising: supplying voltage pulses of aninitial width to the motor using pulse width modulation; comparing amotor current generated due to the supplied voltage pulses to at leastone of three different thresholds; and changing the pulse width from theinitial width to one of zero, a first alternative pulse width and asecond alternative pulse width based on the comparison to a giventhreshold.
 2. The method of claim 1, wherein the alternative width isshortened.
 3. The method of claim 1, wherein the alternative width islengthened.
 4. A power tool, comprising: a housing; a power supplydisposed within the housing and including a plurality of lithium ionbatteries; a motor disposed within the housing and providing torque tothe power tool; and a pulse width modulation controller within thehousing for varying the pulse width of voltage pulses supplied to themotor based on a comparison of the motor current to one of at leastthree different thresholds.
 5. The power tool of claim 4, wherein thelithium ion batteries are rechargeable.
 6. The power tool of claim 4,wherein the pulse modulation controller includes a switch that suppliescurrent to the motor when the switch is turned on.
 7. The power tool ofclaim 6, wherein the pulse modulation controller further includes amicroprocessor that turns the switch on and off.
 8. The power tool ofclaim 7, wherein the pulse modulation controller further includesvoltage measuring circuitry to determine a turn on time for the switch.9. The power tool of claim 8, wherein the voltage measuring circuitrymeasures battery voltage and motor voltage.
 10. The power tool of claim9, wherein power tool further includes circuitry for speed control. 11.The power tool of claim 10, wherein the circuitry for speed controlincludes a potentiometer.
 12. The power tool of claim 11, wherein thevoltage measuring circuitry further measures a voltage across thepotentiometer.
 13. The power tool of claim 6, wherein the pulse widthmodulation circuit further includes a ramp generator that provides aramp signal.
 14. The power tool of claim 13, wherein the pulse widthmodulation circuit further includes a comparator that compares the rampsignal to a measured motor voltage to determine the pulse widthmodulation waveform.
 15. An apparatus for limiting current to a load,comprising: means for supplying power; means for providing torque to aload; means for switchably supplying power to the means for providingtorque from the means for supplying power, the means for switchablysupplying power being actuated according to a pulse width modulationtechnique; and means for controlling the means for switchably supplyingpower based on a comparison of current generated in the apparatus to oneof at least three different thresholds.
 16. The apparatus of claim 15,wherein the means for supplying power includes lithium ion batteries.17. The apparatus of claim 16, wherein the means for supplying powerprovides a direct voltage between 30 and 40 volts.